Search

In order to improve the efficiency of battery charging and discharging in a wide conversion gain, a dual-transformer-based full bridge converter using a block capacitor was proposed. By adding the blocking capacitor in the primary side of one transformer, an extended zero voltage switching (ZVS) range is obtained in a wide output voltage range, which is achieved by regulating the secondary voltage of the transformers to match the output voltage according to the voltage-second balance in both boost and buck modes. Besides, a mirror-symmetrical modulation is introduced in reverse power flow to avoid a limited ZVS range of the traditional dual-transformer converter. The power characteristics and ZVS range are analyzed in detailed. The analysis results and transformer current are compared with the published dual-transformer converter and the conventional dual active bridge (DAB) converter to demonstrate the advantages. Finally, a 300W prototype was built and tested to verify the effectiveness of the proposed converter.

The proliferation of rooftop solar PV has caused reverse power flow in low voltage distribution networks leading to elevated voltage levels. As the voltage is raised, PV inverters reduce output power to combat voltage rise and may result in inverter shutdowns causing PV curtailment (loss of PV output power). Residential battery storage can store excess PV energy and hence reduce reverse power flow, over voltage and PV curtailment. This study aims to determine the reduction in PV curtailment achieved by the installation of optimally sized residential battery storage. A method to calculate the reduction in PV curtailment and the reduction in corresponding financial losses incurred by PV owners and the energy market is presented. The method is applied to an 11-kV feeder in South Australia along with prevailing feeder and inverter voltage limits. Results show that with the installation of optimally sized battery storage, PV curtailment and the corresponding financial losses can be reduced by more than 50%. Refereed/Peer-reviewed

The increasing penetration of distributed generations (DGs) has brought huge challenges to distribution networks operation, such as reverse power flow and overvoltage. Under this background, an optimal active and reactive power dispatch (OARPD) method is proposed in this paper. Considering the three-phase unbalance property, the bus injected model (BIM) of power flow is established while a novel high precision linearized method is proposed. Based on the linearized power flow model, the multi-objective OARPD model which aims to minimize networks losses while maximize the absorption of renewable energy is constructed. Moreover, to ensure the system safe operation and ameliorate the adverse effects caused by single-phase integration of DG, a novel voltage unbalance constraint is established. Finally, examples have been used to demonstrate the validity and effectiveness of the proposed method.

This paper presents an analysis of the system effects of reverse power flow in distribution feeders. Continued increases in the number of small-scale photovoltaic (PV) panel installations within the network has led to low or reverse power flows in distribution feeders at times of high solar energy availability, and substantial weather-induced load variation, among other impacts. In the event of a large loss of generation, under-frequency load shedding of these distribution feeders is one tool that network operators use to arrest the frequency decline. The penetration of distributed PV generation leads to low availability of load to shed, or the chance of inadvertently removing generation, which can lead to power system failure. This paper explores these effects on the largest power system in Western Australia and suggests methods to mitigate the impact of increasing DER penetration on under-frequency load shedding schemes.

Reverse power flow from a photovoltaic (PV) system in a distribution system causes a voltage rise. A relative study regarding the reduction in the distribution feeder voltage depending on system conditions and the magnitude of reverse power flow has been conducted. Several methods for mitigating voltage rise have been proposed; however, the influence of these methods on the voltage in the distribution system, where the voltage is reduced due to reverse power flow, remains to be determined. In this study, the effect of constant power factor control in low-voltage PV systems, which are widely used as voltage rise countermeasures in distribution systems, was analyzed under the condition that the distribution line voltage decreases due to reverse power flow. Consequently, the constant power factor control of the low-voltage distribution system was found to adversely reduce voltage in the medium voltage distribution system due to the consumption of lagging reactive power by the PV systems.

Battery energy storage (BES) has the ability to solve many power system problems especially in systems with renewable energy resources integrated. Due to the intermittent nature of solar photovoltaic (PV) generation, different problems have arise such as voltage fluctuation and reverse power flow. Besides, duck curve issue has become more significant with the increasing amount of PV integrated in the power system as first raised by the Californian Independent System Operator. Studies on these issues has become important in order to solve the steep ramp and over generation problems caused by duck curve issue. The duck curve issues based on general Malaysia load profile and PV pattern are studied in this paper. Besides, the feasibility of BES in solving these issues with suitable BES capacity is investigated. The results show that the BES has the ability to solve the duck curve issues through proper control of BES charging and discharging operation.

Integration of the renewable energy sources (RES) in distribution systems is accompanied with a lot of challenges to power system protection, control and operation. This integration can cause reverse power flow that affects the voltage stability of the power system. To avoid the power system instability, the monitoring of the integration of RES is necessary. In this paper a hybrid algorithm is proposed to monitor reverse power flow, and then it was tested on the IEEE-39 and IEEE-118 node test feeders.

Increasing penetration rate of renewable energy in distribution network (DN) require effective measure for self-balancing. Considers the profit of multi investment entities with distributed energy resources (DERs) in a district DN, this paper proposed an incentive-based optimal operation method. This is a win-win situation for both the utility grid and the investors of DERs. On the one hand, the demand fluctuation and reverse power flow is reduced to delay the investment of gird expansion; the other part is the multi investment entities can benefit from the coordinated response process. The case study demonstrates the effectiveness of the proposed approach.

In this study, we have proposed a novel current injection determination method that improves the voltage unbalance based on the unbalanced line impedance in a distribution network with a large-capacity PV system. An increase in the unbalance of the distribution line voltage was observed owing to a large-scale reverse power flow. To visualize this phenomenon, the P-V curves were derived for each phase to indicate the increase in the voltage unbalance with respect to the reverse power flow. Based on the derived P-V curves, the effect of a current unbalance on the voltage unbalance was investigated. It was clarified that there is a current unbalance that can improve the voltage unbalance even if the line impedance is unbalanced. In other words, the current unbalance that can theoretically make the voltage unbalance zero could be expressed in terms of the symmetrical components of unbalanced line impedance. As an application of the proposed method, the effect of the mitigation of voltage unbalance was demonstrated by controlling single-phase reactors, whose numbers were determined by using the relationship between the unbalanced line current and unbalanced line impedance.

Integration of distributed generation (DG) in existing distribution networks has been studied thoroughly during the past years as a measure of reducing grid's power losses. However, the optimal DG placement, known as ODGP, toward loss minimisation, has not been studied in depth by considering the possible impact of the reverse power flow (RPF) caused by extended penetration of distributed energy resources. This study uses a constriction factor embedded local particle swarm optimisation algorithm along with the appropriate particle formulation that solves the ODGP problem by taking into account the impact of possible RPF. In this study, the idea of RPF modelling is introduced by providing extended versions of IEEE test systems. Modified versions of the IEEE 30-bus and IEEE 33-bus test systems are modelled and results are presented in order to highlight the impact of RPF on the ODGP problem solution. The mathematical formulation is given, results and analysis for both extended systems are presented, while the importance of RPF for different conditions is assessed.

Significant growth in PV penetration worldwide has introduced intriguing challenges for power utilities and consumers alike. This include financial losses resulting from overvoltage-induced PV curtailment during times of high PV generation. This paper examines these issues by first developing a methodical approach to quantify the impacts of PV penetration in terms of reverse power flow, overvoltage and undervoltage events. A real distribution feeder in South Australia is analysed against various PV penetration levels. The simulation model developed utilises residential load profile of South Australia and voltage regulation limits of the feeder and the inverter as set by the Australian standard. Results show that increase in PV penetration reduces the instances of undervoltage, however the instances of overvoltage increase substantially. The latter leads to inverter shutdowns when the voltage exceeds the nominal maximum voltage of the inverters. Maximum possible PV generation loss due to inverter shutdown is evaluated and some recommended solutions are presented. The financial loss due to PV curtailment has been estimated from different perspectives, namely, using feed-in-tariff, retail price and the pool price (market price). In the context of rapidly evolving regulatory frameworks and financial models for renewable energy, power utilities will need to take these financial losses into account when conducting cost-benefit analysis for possible network upgrades for increasing the PV hosting capacity of the network. Refereed/Peer-reviewed

The impact of PV generation distributed in a low voltage transmission line depends on many factors: The distribution lines and PV generators characteristics, its location, operational control, local meteorological conditions, electricity consumption profile, and the electricity cost variation. An atypical and challenging behavior of photovoltaic distributed generation (DG) insertion in consumer units (CUs), implies in some circumstances, as the reverse directionality of the power flow between the load equipped with a photovoltaic system generator and the electrical grid, when a CU contains a distributed generation and low power consumption, the power flow will be directed to the power electric grid. In this work, the modeling of a low-voltage real feeder was performed, setting the variables of the system under real operating conditions. As result, voltage levels variability throughout the feeder, the electrical losses, and the asymmetry between the phases were observed. Through simulation scenarios, the occurrence of voltage increase under different penetration scenarios of distributed generation was verified and there was a 10% increase in reference voltage as well as the occurrence of higher electrical losses by reverse current, reaching 1200% more with a DG penetration, in the massive presence of the photovoltaic generator. The mitigatory action used in this work was able to attenuate the negative impacts to the feeder circuit, ensuring the integrity grid and the consumer unit.

Submitted by Thayannã Yury Furtado Gadelha (thayanna.yury@unesp.br) on 2019-10-17T04:42:25Z No. of bitstreams: 1 Dissertação Thayannã Yury Furtado Gadelha.pdf: 1878900 bytes, checksum: 65d6ef18b1795ef51545803c15939bb3 (MD5) Approved for entry into archive by Cristina Alexandra de Godoy null (cristina@adm.feis.unesp.br) on 2019-10-17T12:21:11Z (GMT) No. of bitstreams: 1 gadelha_tyf_me_ilha.pdf: 1878900 bytes, checksum: 65d6ef18b1795ef51545803c15939bb3 (MD5) Made available in DSpace on 2019-10-17T12:21:11Z (GMT). No. of bitstreams: 1 gadelha_tyf_me_ilha.pdf: 1878900 bytes, checksum: 65d6ef18b1795ef51545803c15939bb3 (MD5) Previous issue date: 2019-10-08 Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) Devido a um conjunto de questões econômicas e ambientais, a geração distribuída baseada em fontes renováveis tem sido incentivada por vários países. No Brasil, a conexão de unidades de geração distribuída fotovoltaica (GDFV) às redes de distribuição de baixa tensão, por meio de unidades residenciais e comerciais, tem apresentado uma rápida expansão. Diante deste cenário, é necessário investigar os possíveis impactos negativos que tal expansão pode resultar sobre os sistemas de distribuição. Neste trabalho, busca-se avaliar o impacto sobre a tensão em regime permanente. Para se alcançar tal objetivo, utiliza-se como modelo do sistema de distribuição a rede de teste IEEE 13 barras. Através do software de acesso livre OpenDss, o qual é devidamente apresentado, o comportamento da tensão ao longo desta rede, antes e depois da inserção da GDFV, é estudado por meio de simulações pontuais e diárias. Também é analisado neste trabalho de que forma o sistema de geração distribuída fotovoltaica, através do uso do controle voltvar, pode atenuar tal impacto. Due to a set of economic and environmental issues, distributed generation based on renewable sources has been encouraged by several countries. In Brazil, the connection of photovoltaic distributed generation units (PVDG) to low voltage distribution networks through residential and commercial units has shown a rapid expansion. Given this scenario, it is necessary to investigate the possible negative impacts that such expansion may result on distribution systems. This work aims to evaluate the impact on the steady state voltage. To achieve this goal, the IEEE 13 bus test network is used as a model of the distribution system. Through the open access software OpenDss, which is properly presented, the behavior of the voltage along this network, before and after the insertion of the PVDG, is studied through punctual and daily simulations. It is also analyzed in this work how the photovoltaic distributed generation system, by voltvar control, can mitigate such impact. CAPES: 001

This paper proposes the application of three modelling techniques for analysis of bidirectional resonant converters with capacitor-inductor-inductor-capacitor (CLLC) compensation network operating under frequency modulation. Fundamental Mode Approximation (FMA), state-variable and cyclic-averaging models are obtained and compared. The simulation results for forward and reverse power flow direction are verified against Spice simulation and results obtained from literature. All models could successfully predict the behavior of the converter but state-variable and cyclic-averaging techniques showed better accuracy. Execution time is compared confirming that cyclic-averaging is the method with fastest execution.

The growth of photovoltaic systems (PV) and battery energy storage systems (BESS) will become a challenge for distribution players. In this way it is important to understand how these systems can impact on the distribution grid operation. For this, a study was developed considering the insertion of PV on IEEE 13 buses test feeder, based on different penetration levels. These levels were calculated contemplating two different parameters: the one most used in Brazil, and the parameter that consists in an extended criterion used worldwide. Considering these possibilities, different penetration levels of photovoltaic were defined. In conjunction with photovoltaic generation, a BESS was inserted in the same feeder, in order to evaluate the impacts of this system. From the simulations was possible to evaluate the tap changes, the voltage profile, power losses and the total energy, including if there was reverse power flow at any moment during one day.

In the last decade, there has been a tremendous growth in the field of grid-tied Photovoltaic (PV) generating plants in INDIA. With the recent government subsidies and reduction in the panel prices, the generation capacity has increased from a few kilowatts to hundreds of Mega-Watts. This rapid escalation in the penetration level on the grid has increased the utility companies’ concerns about its impact on the power quality of the system.In the current scenario, the penetration levels of the Photovoltaic generating in INDIA is very low to create any kind of significant power quality issues on the grid, however, power quality issues such as current harmonics and power factor have been identified in some cases. But with the increase in the penetration levels of PV generator on the grid, it may start affecting the overall power quality of the system. Power system stability and security issues, voltage variation (i.e. sag & swell), reverse power flow, power factor, and harmonics are the major power quality issues that may arise due to increased PV penetration. This paper gives an overall review of different power quality issues that may arise due to the increased penetration level of a PV generator on the grid.

Negative impacts of large-scale rooftop photovoltaic (PV) penetration may cause performance deterioration of distribution grids. Consequently, a rooftop PV hosting capacity analysis is becoming a concern of utility. In this work, a proposed method based on Monte Carlo process is developed to assess the PV hosting capacities on the basis of substation power factor. To consider the actual conditions, three selected feeders including feeder F1, feeder F2, and feeder F3, with distinct grid characteristics from Yogyakarta Province of Indonesia are employed. To obtain a detailed result of the rooftop PV hosting capacity, the customer with rooftop PV is randomly selected one by one. The proposed method is performed with over/undervoltage, substation power factor and reverse power flow limit. The comparative study is provided to investigate the impacts of the operational limits on the rooftop PV hosting capacity. Since the load demand and PV output vary with the time, the analyses are conducted in a time-series framework. In this work, constant PQ of loads is assigned, and all rooftop PV systems are assumed to have unity power factor. The results show that the substation power factor limit is more restrictive compared to overvoltage limit and reverse power flow limit. The substation power factor limits the rooftop PV penetration on feeder F1, feeder F2, and feeder F3, respectively, 86.52%, 79.37%, and 85.85% faster than reverse power flow limit. The results also indicate that the substation power factor decreases with the increasing of rooftop PV penetration.

The increasing amount of power plants connected to the distribution networks is pointing out issues about energy balances of local communities as well as observability and management of the energy flows by the Distribution System Operator. This paper presents a real life Demand Response campaign carried out by the local Distribution System Operator to reduce the Reverse Power Flow caused by the increasing amount of energy produced locally by distributed generators from not programmable energy sources. Innovative near-real time smart meters and advanced software developed under the Nobel GRID and the WiseGRID projects were used for the trial. Furthermore, these findings and actual data have been used for running simulations at the primary substation level for a complete year. The encouraging results obtained, especially when large energy consumers are involved, have shown the effectiveness of these tools and how they can enable the Distribution System Operator to use Demand Response strategies for increasing grid efficiency and control.

This paper proposes a decentralized charging control method of a battery energy storage system (BESS) that can prevent the over-capacity of a pole transformer in low-voltage (LV) distribution systems due to reverse power flow from the photovoltaic generation system. The distribution system operator provides the charging parameters defined as the relationship between the state-of-charge (SoC) and the charging threshold determined by using only the LV-distribution system configuration and the demand-side information to customers. Each BESS charges the power according to the provided parameters. A high charging amount can be maintained by automatically changing the charging mode based on the change in SoC. Results of the numerical simulation demonstrate that the proposed method can prevent the over-capacity of PTr and maintain the high charging amount of a BESS in LV-distribution systems. Further, the maximum power flow at the distribution substation and the distribution losses in MV-distribution system can be reduced.

DC-feeding Microgrid (DCMG) which can integrate plural DC-driven facilities by using information is considered as a future energy system for rural shore area. To balance supply and demand in the DCMG, a set of fuel cells and electrolyzer (FCE) systems is proposed as an energy storage system instead of conventional battery systems. DCMG including FCE, PV and DC loads are assumed, the mode controlling method for FCE to balance supply and demand is proposed in this paper. As a result, FCE is controlled by proposed method and can balance supply and demand without reverse power flow to utility grid.

In this paper a novel approach regarding the optimal penetration of Distributed Generation (DG) in Distribution Networks (DNs) towards loss minimization is proposed. More specific, a Local Particle Swarm Optimization (PSO) variant algorithm is developed in order to define the optimal active and reactive power generation and/or consumption requirements for the optimal number and location of nodes that yield loss minimization. Thus, the proposed approach provides the optimal number, siting and sizing of DGs altogether. In addition, based on the optimal power requirements of the resulted nodes, a combination of potential DG types to be installed is recommended. The proposed objective function in this paper is also innovative since it embeds the constraint of reverse power flow to the slack bus by the formation of a new penalty term. The proposed methodology is applied to 30 and 33 bus systems. The results indicate the optimal number, locations, and capacity of DG units, which were calculated simultaneously. Finally, the impact of the predefined amount of permissible reverse power flow to the optimal solution is also examined through two scenarios: the first considers zero reverse power flow and the second unlimited reverse power flow.

Deployment of high-penetration photovoltaic (PV) power is expected to have a range of effects – both positive and negative – on the distribution grid. The magnitude of these effects may vary greatly depending upon feeder topology, climate, PV penetration level, and other factors. In this paper we present a simulation study of eight representative distribution feeders in three California climates at PV penetration levels up to 100%, supported by a unique database of distributed PV generation data that enables us to capture the impact of PV variability on feeder voltage and voltage regulating equipment. We find that feeder location (i.e. climate) has a stronger impact than feeder type on the incidence of reverse power flow, reductions in peak loading and the presence of voltage excursions. On the other hand, we find that feeder characteristics have a stronger impact than location on the magnitude of loss reduction and changes in voltage regulator operations. We find that secondary distribution transformer aging is negligibly affected in almost all scenarios.

The number of installed photovoltaic (PV) systems has been increasing in an unprecedented rate every year throughout the world. In some situations, the PV systems can provide more power than the demand in certain parts of a network and cause reverse power flow, which the traditional distribution networks are not designed for. Consequently, these instances may result in adverse interactions between PV systems and voltage regulation devices such as step voltage regulators (SVRs). This can potentially drive voltage magnitudes beyond acceptable limits and possibly damage consumers’ appliances. In the literature, such interaction cases with consideration of realistic three-phase four-wire unbalanced networks have not yet been reported. Therefore, this study analyses the interaction between SVR and PV systems in a real-world network by utilising the quasi-static time-series technique for a long-term statistical assessment. This evaluation focuses on examining voltage regulation issues in an unbalanced three-phase four-wire network along with an open-delta SVR configuration, and the corresponding solution is proposed to resolve the concerning issues. The results of this study will provide valuable information on interaction characteristics to academia and utilities for assessing large-scale PV system integration.

This paper investigates and analyzes the impacts of reverse power flow on the power system connecting elements (transformers and cables). For effective investigation and analysis, the paper groups the network under investigation into Zones A and B. Zone A is directly connected to a wind farm. Zone B is far from the wind farm. Different wind energy penetration levels are simulated. The results show that the elements experience reverse power flow at different penetration levels depending on their location and distance from the point of common coupling. Reverse power flow incurs additional element losses. In Zone A, loading and losses increase with reverse power flow, while Zone B experiences loss reduction with increase in reverse power flow. The elements operate within acceptable thermal ratings.

Utility-scale renewable energy systems are rapidly increasing in the market and converting conventional power supply networks to sustainable power supply networks (SPSNs). This requires the redesign of the configuration of these networks for optimal and reliable operation. In this paper, we tackle this problem by considering switches in an SPSN and a new model of utility-scale photovoltaic stations (PVSs). In addition, export power, i.e., reverse power flow to a connected external network is considered. An optimization approach called one-at-a-time-search is adapted and used to solve the formulated mixed-integer nonlinear programming (MINLP) problem. The objective is to minimize the power loss and meanwhile to minimize the curtailed power generated from the PVSs. A 5-bus test SPSN is used to show that circa 40 % of energy losses can be saved.

Above the 10% penetration level most distribution networks are more adversely affected by the power flow and frequency problems that integrated PV systems create due to solar radiation intermittence. Several simulation-based solutions have been proposed to mitigate these negative impacts. The networks on which the solutions are tested make use of maximum fixed distribution network loads which differ from the actual network demand which is time variant. This time variant nature is of interest in residential networks with integrated PV systems because peak insolation is received during the afternoon when load demand is low, thereby creating potential reverse power flow scenario which would otherwise go unnoticed if fixed maximum demand loads are used. This paper proposes that the difference in actual and simulated PV penetration levels, does in part, result from the use of fixed loads and assesses the technical implications.

Dual-transformer-based dual active bridge (DAB) converter with wide ZVS range for wide voltage conversion gain application is derived and studied in this chapter. The phase shift control is adopted for this converter with minimum power switches employing half-bridge output, and a control law is proposed to achieve wide ZVS range. With the proposed method, four switches of the converter can achieve full range of ZVS. The other two switches can achieve full-range ZVS under positive power flow, while slightly reduced ZVS region under reverse power flow. Unlike the methods employing three control degrees of freedom, the proposed method only utilizes two decoupled control variables, making the controller easy to be implemented. In addition, the design of turns ratios for the two transformers under the proposed control is also optimized based on the gain ranges and conduction loss. The effectiveness of the converter with the proposed control is verified by experimental results from a 1 kW prototype.